(1) Field of the Invention
The present invention relates to methods for fabricating high-reliability condenser microphones, which allow the condenser microphones to be reduced in size, and condenser microphones.
(2) Description of Related Art
Known condenser microphones (acoustic sensors) have been each constructed such that a sound wave detector and a detection circuit are formed on individual substrates and the substrates are packaged with insulating spacer layers interposed therebetween (see, for example, Japanese Unexamined Patent Publication No. 2003-163996).
FIG. 6 is a cross-sectional view illustrating the structure of an electret condenser microphone (ECM) described in Japanese Unexamined Patent Publication No. 2003-163996.
As illustrated in FIG. 6, an ECM 100 includes a composite diaphragm (vibrating electrode) chip 101, a fixed electrode chip 102 and a case 103. Among them, the composite diaphragm chip 101 includes a diaphragm 105, a first metal electrode 108, an electret layer 109, and insulating spacers 110. The fixed electrode chip 102 includes a second metal electrode 117, air holes 115, an air chamber 116, and a MOS field effect transistor (MOSFET) 112. The composite diaphragm chip 101 and the fixed electrode chip 102 are arranged so as to be opposed to each other and packaged by the case 103 with the insulating spacers 110 interposed therebetween. The second metal electrode 117 of the condenser microphone is connected through a wire to the MOSFET 112. A voltage variation associated with a variation in the capacity between a diaphragm electrode and a fixed electrode is detected by the MOSFET 112.
The composite diaphragm chip 101 and the fixed electrode chip 102 are fabricated using individual substrates as illustrated in FIGS. 7A, 7B, and 8A through 8C.
The composite diaphragm chip 101 is fabricated in the following manner. First, as illustrated in FIG. 7A, a diaphragm 105 made of a silicon nitride film is formed on the back surface of a first substrate 104, and an insulating film 106 is formed on the top surface thereof. Thereafter, a recess 107 is formed in the insulating film 106 and the first substrate 104. Then, as illustrated in FIG. 7B, a first metal electrode 108 is formed to cover the insulating film 106 and the recess 107, and an electret layer 109 is formed on the back surface of the diaphragm 105. Thereafter, insulating spacers 110 are formed on the back surfaces of lateral end parts of the electret layer 109.
The fixed electrode chip 102 is fabricated in the following manner. As illustrated in FIG. 8A, a MOSFET 112 is formed on an end part of a second substrate 111. Next, as illustrated in FIG. 8B, insulating films 113 are formed on the top and back surfaces of the second substrate 111, and a plurality of air holes 115 are formed in the top surface of the second substrate 111. Thereafter, as illustrated in FIG. 8C, an air chamber 116 is formed in the back surface of the second substrate 111, and a second metal electrode 117 is formed on the top surface of the second substrate 111.
The composite diaphragm chip 101 and fixed electrode chip 102 so fabricated are arranged so as to be opposed to each other as illustrated in FIG. 6 and packaged by a case 103 with insulating spacers 110 interposed therebetween. In this way, an ECM 100 is completed.